1. Field of the Invention
The present invention relates to a method and an apparatus for tightening, fastening or clamping a workpiece between an externally threaded member and an internally threaded member, by rotating a rotary threaded member which is one of these two threaded members, while the rotary threaded member and a stationary threaded member which is the other threaded member are held in engagement with each other, so as to prevent rotation of the stationary threaded member. More particularly, the present invention is concerned with techniques for improving the accuracy of control of tightening or clamping force of the workpiece between the two threaded members.
2. Discussion of the Related Art
A workpiece or a plurality of workpieces may be tightened by and between externally and internally threaded members. For example, one workpiece having at least one through-hole may be fastened to an internally threaded member by screwing at least one externally threaded member each in the form of a bolt into the internally threaded member such that each bolt extends through the corresponding through-hole. Further, a plurality of workpieces may be tightened or clamped together by using at least one headed bolt and at least one nut as the externally and internally threaded members. A workpiece or a plurality of workpieces may be fastened to a member having at least one stud bolt fixed thereto, by screwing a nut on each stud bolt. In this instance, the member having the stud bolt or bolts is considered to be the externally threaded member, and the plurality of workpieces are fastened together in most cases. For tightening, fastening or clamping operation, either of the externally threaded member and the internally threaded member is rotated, and the other threaded member must be prevented from being rotated. In the following description, the threaded member to be rotated will be referred to as "rotary threaded member" while the other threaded member whose rotation is prevent will be referred to as "stationary threaded member".
For tightening or clamping the workpiece or workpieces by the externally and internally threaded members, the axial force acting on the externally threaded member must be accurately controlled as the tightening force. Since it is difficult to accurately detect this tightening force, however, the tightening force is conventionally controlled according to a "calibrated wrench tightening method", a "turn of nut tightening method" or a "torque gradient method" according to JIS B 1083 "Screw Tightening Method". In the "torque gradient method", the accuracy of control of the tightening force is not influenced by a friction coefficient between thread surfaces of the externally and internally threaded members (hereinafter referred to as "thread surface friction coefficient") and a friction coefficient between seat surfaces of the rotary threaded member and the workpiece (hereinafter referred to as "seat surface friction coefficient"). In the "calibrated wrench tightening method"and "turn of nut tightening method", the accuracy of control of the tightening force is influenced by those friction coefficients. Conventionally, the thread surface friction coefficient and the seat surface friction coefficient are measured before the tightening operation, so that the tightening force is controlled based on the measured friction coefficients. However, these two friction coefficients are influenced by the states of the thread surfaces and the seat surfaces, namely, by the surface roughness, hardness and lubricating state of the thread and seat surfaces, which are not usually constant. Accordingly, the tightening force tends to have a considerably large variation, leading to insufficient reliability of control of the tightening force.
Usually, at least one of a spring washer and a plane washer is interposed between the seat surface of the rotary threaded member and the seat surface of the workpiece. Accordingly, slipping usually takes place between the rotary threaded member and the washer or washers during rotation of the rotary threaded member. In the present application, the washer or washers is/are considered as part of the workpiece, and the seat surface of the washer(s) is considered as the seat surface of the workpiece. That is, the presence of the washer or washers is ignored in the present application. It is also noted that the spring washer may be rotated with the rotary threaded member. In this case, the seat surface of the spring washer contacting the seat surface of the workpiece should be considered as the seat surface of the rotary threaded member. Practically, however, it does not cause any problems in this case, to consider that the slipping takes place between the rotary member and the spring washer.